1. Field of Invention
The present invention relates to thermoplastic vulcanizates and methods of forming thermoplastic vulcanizate to improve processability and extrusion properties.
2. Description of the Related Art
Thermoplastic vulcanizates (“TPV”) consist of an amount of finely dispersed crosslinked rubber particles in a continuous thermoplastic. Dynamic vulcanization is the process of selectively crosslinking rubber during its melt mixing with a molten thermoplastic and the best method to produce TPVs. TPVs have the benefit of the elastomeric properties provided by the rubber phase, with the processability of thermoplastics.
The rubber phase can be comprised of more than one rubber component so long as they are vulcanizable as well. Additional uncrosslinked thermoplastics, including uncrosslinked elastomers, can as well be included in the continuous thermoplastic phase, and can in some instances additionally be partially incorporated in the crosslinked rubber phase, thus creating interfacial adhesion and increased compatibility between the continuous and discontinuous phases. Process oils, in addition to any extender oils in the rubber, or other polymeric components, are optionally introduced to improve processing at various locations in the processing equipment. Traditional fillers, reinforcing fibers, additives, colorants, and the like, can be added before or after extrusion of the TPV compositions from the melt processing extruder. Typically, the TPV compositions described can be pelletized by methods commonly used in the thermoplastics processing art. The TPV pellets so prepared can be further melt processed with other conventional components or additives in compounding, for thermoplastic molding or shaping into final products.
The environment in which thermoplastic vulcanizates are formed and in which vulcanization of the rubber constituent occurs is typically defined by significant shearing forces, heat, and the presence of a variety of additives, including rubber curing agents and co-agents that facilitate cross-linking of the rubber. The processing conditions and selection of materials in the TPV composition can materially impact the quality of the TPV on extrusion. It is desirable to provide TPVs that exhibit good physical properties and processability, while maintaining excellent extrusion properties. However, balancing these desirable characteristics has proven difficult. Extrusion surface smoothness (which may also refer to as extrusion surface roughness (ESR)) is a particularly important extrusion property as the ESR may dictate the suitability and aesthetics of a final extruded product. Thus, it is desirable when formulating and processing TPVs to maintain or improve ESR in combination with efforts to improve physical properties and processability.
Exemplary of the impact that processing conditions, for example, can have on the physical and extrusion properties of thermoplastic vulcanizates, it is known that peroxide curing agents can degrade polyolefin thermoplastics, most notably polypropylene, in such a way as to adversely affect the physical properties of the resultant thermoplastic vulcanizate. The degradation of polypropylene in peroxide cured TPVs has created technical challenges that have given rise to new approaches of forming peroxide cured polypropylene based TPVs so as to protect the polypropylene from degradation in order to improve physical and extrusion properties.
U.S. Pat. No. 6,610,786 and US 2005/256273 describe a process for making a TPV that involves adding a portion of the thermoplastic and selected other additives to the composition after the peroxide curing agent has been added to the extrusion chamber and the rubber has been substantially cured. The Mitsui approach has the benefit of protecting a portion of the thermoplastic from degradation by the peroxide curing agent; however, on extrusion of the material from the extrusion chamber, the TPV has less than desirable extrusion surface smoothness.
Another approach described in US 2004/236032 involves a process of making a master batch by melt blending the elastomer with the thermoplastic polyolefin, curing co-agents and selected additives. The master batch, which is uncured, is solidified and then melt mixed with a curing agent and optionally other additives. Thus, Basell produces an uncured master batch containing the full amounts of elastomer and thermoplastic polyolefin. Curing takes place in a second processing step. However, the thermoplastic polyolefin (i.e., polypropylene) is subjected to exposure by the curing agent in the second processing step, thereby adversely affecting physical properties.
It would be desirable to provide a method of forming thermoplastic vulcanizates to balance physical properties and processability, while maintaining excellent extrusion properties, including extrusion surface smoothness.